In a diesel engine of this type, a severe emission control has been imposed year to year in accordance with a recently increased attention to environmental issues. In particular, NOx and a particulate matter (hereinafter abbreviated as “PM”) among harmful substances contained in exhaust gas have been required to be reduced.
However, NOx is liable to be generated in a complete combustion state: in contrast, the PM is liable to be generated in an incomplete combustion state. Therefore, NOx and the PM have the relationship of a trade-off: namely, one is decreased in exhaust amount while the other is increased in exhaust amount. As a consequence, the simultaneous reduction of the exhaust amount of both of NOx and PM is an important problem to be solved in the field of the diesel engine.
Examples of means conventionally adopted to reduce NOx include an EGR (abbreviating an exhaust gas recirculation) for circulating a part of exhaust gas in an intake system, retardation of a fuel injection timing, and the like. However, only such means induces an increase in PM due to the relationship of the above-described trade-off.
The shape of a combustion chamber formed at an upper portion of a piston is significantly involved in the increase in PM, as described below.
For example, Patent Literature 1 discloses a direct-injection diesel engine, in which a recessed combustion chamber is formed at an upper portion of a piston. As shown in FIG. 10, a combustion chamber 101 is configured by including a conical center projection 102 at the center of a bottom and forming an annular groove 103 into a substantially arcuate shape, as viewed in cross section, around the center projection 102. An opening 104 of the combustion chamber 101 is formed into a circular shape. A lip 105 protruding toward an inner circumference in such a manner as to narrow an opening area is formed is formed at the opening 104. Fuel is radially injected toward the inside of the combustion chamber 101, to be then mixed with air in the combustion chamber 101, followed by combustion.
A flame or a mixture air generated in the combustion chamber 101 flows from the combustion chamber 101 to a main chamber 106 in an expansion stroke. At this time, if the opening of the combustion chamber is formed into the circular shape, the flame or the mixture air uniformly flows in a circumferential direction, thereby weakening the flow of the flame or the mixture air from the combustion chamber 101 to the main chamber 106. Moreover, the fuel injected from a fuel injection nozzle is allowed to swirl inside of the combustion chamber 101 in the circumferential direction while being mixed with the air in the combustion chamber 101. Here, since the flow outside of the combustion chamber 101 is weak in the expansion stroke, the flame or the not-burnt mixture air flows inside of the combustion chamber 101 in the circumferential direction, thereby prolonging a staying time inside of the combustion chamber 101. As a consequence, the mixture of the fuel and the air cannot be promoted in, particularly, the main chamber 106, and therefore, combustion in a late period cannot be properly performed.
Under the above-described circumstances, even if the fuel injection timing is retarded for the purpose of NOx reduction, a retardation limit cannot be extended since there is a high possibility of an increase in amount of black smoke due to the incomplete combustion. Furthermore, an air utilization rate becomes low even at a high EGR rate, and therefore, a combustion speed becomes low, resulting in the increase in amount of black smoke.
Prior art literature is exemplified by Japanese Patent Application Laid-open (JP-A) No. 2001-221050.